Field of the Invention
The present invention relates to a circuit configuration for producing an at least approximately ideal diode characteristic on the basis of a diode.
The function of a diode is used in a large number of practical applications. For example, a diode may be connected in antiseries with a power MOSFET, in order to prohibit current in the reverse flow direction in a circuit containing a power MOSFET which permits current to flow in the reverse direction.
A diode is, for example, configured as a PN diode, which produces a large power loss because of the required bias voltage or threshold voltage. That problem of a PN diode can, for example, be mitigated through the use of a Schottky diode, but not fully eliminated. Furthermore, a disadvantage of Schottky diodes is that they have high leakage currents at high temperature.
In other words, diodes have a characteristic which deviates from the ideal of a resistor characteristic due to the threshold voltage, and therefore unavoidably result in a power loss.
It is accordingly an object of the invention to provide a diode circuit with an ideal diode characteristic, which overcomes the herein afore-mentioned disadvantages of the heretofore-known devices of this general type and with which a diode characteristic matched to a profile of a resistor characteristic can be produced for a diode.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for producing an at least approximately ideal diode characteristic, comprising a diode having a forward-bias direction and a reverse-bias direction; and a power MOSFET having a control path connected in parallel with the diode, a load path forming ideal diode connection terminals, and a gate connection receiving a predetermined voltage potential for turning on the power MOSFET in the forward-bias direction of the diode and turning off the power MOSFET in the reverse-bias direction of the diode.
In accordance with another feature of the invention, there is provided a Zener diode connected between the gate connection of the power MOSFET and the diode.
The invention thus proposes to overcome the problem of the diode threshold voltage by using a power MOSFET which is connected in parallel with the diode in the third quadrant. By virtue of this measure, the output characteristic of the diode, or of this diode circuit, is similar to the characteristic of a resistor. Therefore, the voltage drop across the diode, i.e. the product of current times resistance, can be considerably smaller with suitable dimensioning than the diode threshold voltage of a conventional or conventionally used diode. This advantageously leads to a reduction in the power loss.
It will be understood that the power MOSFET used according to the invention in the diode circuit must have an activation threshold voltage. The solution according to the invention is turning the power MOSFET on in the forward-bias direction of the diode and turning it off in the reverse-bias direction, through the use of which a virtually ideal diode characteristic can be obtained.
The invention also provides a measuring instrument based on the diode circuit according to the invention for measuring a load current without the disadvantage of a shunt resistor which was heretofore needed for that purpose. Therefore, in accordance with a further feature of the invention, there is provided a supply voltage source having terminals, and a resistor connected between one of the terminals of the supply voltage source and the gate connection of the power MOSFET. This proposal for such a measuring instrument has the advantage of considerably reducing costs, power loss, number of components required, and overall size and weight of the measuring instrument compared with the prior art.
In accordance with an added feature of the invention, the power MOSFET has a source and a drain, and the drain of the power MOSFET is connected to a voltage reference point; the diode has two electrodes, and one of the electrodes of the diode is connected to the voltage reference point; a Zener diode is connected in antiseries between the other of the electrodes of the diode and the gate connection of the power MOSFET; a supply voltage source has a terminal connected to the source of the power MOSFET; a resistor is connected between the Zener diode and the terminal of the supply voltage source; and optionally there is provided a series circuit having a load and a switching device, the series circuit connected between the source of the power MOSFET and the terminal of the supply voltage source.
In accordance with an additional feature of the invention, there is provided a further MOSFET forming current sensor cells for determining a load current flowing through the load, the further MOSFET having a drain connected to the voltage reference point connected to the power MOSFET, a gate connected to the gate connection of the power MOSFET, and a source; an NMOS transistor having a source connected to the source of the further MOSFET, a drain connected to the terminal of the supply voltage source, and a gate; and a differential amplifier having an output connected to the gate of the NMOS transistor, one input connected to the source of the power MOSFET, and another input connected to the source of the further MOSFET.
In accordance with yet another feature of the invention, there is provided a PMOS current mirror circuit supplying the drain of the NMOS transistor with current.
In accordance with yet a further feature of the invention, there is provided a rectifier diode connected in series with the PMOS current mirror circuit.
In accordance with a concomitant feature of the invention, the further MOSFET forming the current sensor cells and the power MOSFET are made on the same substrate.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a diode circuit with an ideal diode characteristic, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.